Method of tunning wettability of titanium dioxide layers against water

ABSTRACT

The present invention relates to a method of tunning wettability of titanium dioxide layers against water by nanostructuring the titanium dioxide layers to increase a hydrophilicity of the titanium dioxide layers, and also coating the nanostructured titanium dioxide layers with silane layers to increase a hydrophobicity of the titanium dioxide layers. The method of tunning wettability of titanium dioxide layers against water according to the present invention comprises: (a) step of forming titanium dioxide layer on a substrate; (b) step of forming silica particle layers on the upper part of the titanium dioxide layer; (c) step of etching a surface of the laminate prepared in step (b); and (d) step of removing the silica particle layer etched and remained in the step (c).

CROSS-REFERENCES TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. 119(a) to Korean application number 10-2012-0098413, filed on Sep. 5, 2012, in the Korean Intellectual Property Office, which is incorporated by reference in its entirety as if set forth in full.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a method of tunning wettability of titanium dioxide layers against water, and more specifically a method of tunning wettability of titanium dioxide layers against water by nanostructuring the titanium dioxide layers to increase a hydrophilicity of the titanium dioxide layers, and also coating the nanostructured titanium dioxide layers with silane layers to increase a hydrophobicity of the titanium dioxide layers.

2. Related Art

The wettability means the extent of wetting easiness of any liquid against a solid face, and if the wettability is good, the surface tension of liquid is low and thus, the liquid is well-spread on the solid surface, and if the wettability is bad, the surface tension of the liquid is high and thus the liquid is not well spread on the solid surface.

Titanium dioxide (TiO₂) known as a nontoxic material is used as many uses in various fields. In a photocell device, TiO₂ is a main component of a fuel-sensitive solar cell, and n-type of TiO₂ thin-film is used together with noble metals to recover a hot electron in nanodiode. Also, TiO₂ is used in a hydrogen generation or water splitting, and is used as the support for a catalyst reaction.

Since TiO₂ has a photocatalyst property that it becomes a super-hydrophilicity state when UV is irradiated and also becomes a super-hydrophobicity upon bonding with the self-assembling monolayer, and thus, is applied to self-cleaning. Therefore, a number of studies are being made on the relation between a water contact angle and a surface morphology for a wetting action on the surface of the material.

A roughness is a major factor in the wettability control, and was quantitatively defined by Wenzel and Cassie-Baxter's equation. Thus, a plan or approach is sought to roughen the texture of a surface by using various methods including an etching, lithography, particle coating technology, etc. in order to create or mimic superhydrophobic biosurfaces in nature.

An effective method for the nanostructuring includes a colloid lithography using a self-assembling microsphere or nanosphere. This self-assembling microsphere or nanosphere layer is used as a mask or template, and is combined with the nano-manufacturing techniques consisted of a deposition, evaporation, etching, and the like to form an ordered nanostructure. Silicon dioxide and polystyrene particles are the conventional components forming self-assembling 2D colloid monolayer prepared by various conventional methods such as drop-casting, dip-coating, spin-coating, or more specific technologies such as a confined convection assembly, Langmuir-Blodgett trough or electric field. Such approach makes a nano technology easy and can create a simple but an interesting nanoscale structure in a relatively low cost.

The inventors of the present invention developed the method for tuning the wettability of titanium dioxide layers against water by making the hydrophilicity of titanium dioxide layers or the hydrophobicity of titanium dioxide layers via the modification of the surface roughness of titanium dioxide layers, i.e., the roughness change of titanium dioxide layers, and completed the present invention.

SUMMARY

It is an object of the present invention to provide a method for tuning wettability of titanium dioxide layers against water via a surface modification of titanium dioxide layers.

Other purpose of the present invention is to provide a titanium dioxide structure comprising titanium dioxide layers in a shape of nanocone array being uniformly arranged.

Another purpose of the present invention is to provide a titanium dioxide structure having an improved hydrophobicity since it comprises a silane layer.

Upon using the method of tuning wettability of titanium dioxide layers against water, the hydrophilicity and hydrophobicity of titanium dioxide layers can be easily controlled depending on the circumstances.

Therefore, upon utilizing the present invention, the titanium dioxide structures having an improved hydrophobicity as well as the titanium dioxide structures having an improved hydrophilicity can be prepared.

Titanium dioxide structures prepared in the present invention can be manufactured in a large scale because the titanium dioxide layers having an improved hydrophilicity or hydrophobicity can be formed on the large scale of the substrate.

Titanium dioxide layers that the hydrophilicity and hydrophobicity are controlled according to the present invention represent the photocatalyst property and self-cleaning property, and thus, can be applied to a multifunctional transparent film.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of the subject matter of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a mimetic diagram of the procedure for controlling wettability of titanium dioxide layers against water;

FIG. 2( a) represents π-A isotherm of SiO₂ bead monolayer prepared by using Langmuir-Blodgett trough, and FIG. 2( b) is a top view of SEM image of silica nanosphere monolayer;

FIG. 3 is the top and cross-sectional views of SEM image over an etching time of sputtering method TiO₂ layer, wherein (a) is a photograph at the time of unetching, (b) is that at 10 sec etching, (c) is that at 15 sec etching, (d) is that at 20 sec etching, (e) is that at 25 sec etching and (f) is that at 30 sec etching;

FIG. 4 is a graph representing the water contact angle and etching time of TiO layer of (a) before and (b) after the silanation by using 1H,1H,2H,2H-perfluorooctyltrichlorosilane;

FIG. 5 is the cross-sectional and top views of SEM image and photoimage of water contact angle after silanation of 20 sec etching TiO₂ thin-film, wherein (a) is sol-gel method, (b) is a sputtering method, (c) is a thermooxidation method TiO₂ layer;

FIG. 6 represents inclined wall structure upon 20 sec and 30 sec etching of TiO₂, wherein (a) and (c) are schematic diagrams of the wall structure upon 20 sec and 30 sec etchings of sputtering and thermooxidation TiO₂ layer, and (b) and (d) are SEM images upon 20 sec and 30 sec etchings of sputtering TiO₂ layer; and

FIG. 7 represents XPS spectrum of etched TiO₂ layer, wherein (a) and (b) are Ti component and F component of 20 sec etching sol-gel TiO₂ layer, respectively, and (c) and (d) are XPS spectrums of Ti component and F component of 40 sec etching thermooxidation TiO₂ layer.

DETAILED DESCRIPTION

Hereinafter, a method of tuning wettability of titanium dioxide layers against water of the present invention will be described in more detail.

FIG. 1 depicts a mimetic diagram of the procedure for controlling wettability of titanium dioxide layers against water.

At first, titanium dioxide layers are formed on the substrate conventionally used in the art of this field. In the present invention the titanium dioxide layers are formed by a sol-gel method, sputtering method or thermooxidation method conventionally used in the art of this field.

Next, a silica particle layer is formed on the upper part of the titanium dioxide layer. The silica particle layer in the present invention is preferable to be formed by the Langmuir-Blodgett process.

Langmuir-Blodgett process is the method for forming the ordered monolayer film by locating the material to be formed on the surface between water and air in a thin-film and collecting it by a foreign force.

The size of silica particle that can be used in the present invention is 50˜500 nm in diameter. If the size of the silica particle is less than 50 nm, it is difficult to construct the self-assembling monolayer, and also since the erosion of the silica layer is so fast in the etching procedure to be conducted later that there is a problem that the production of nanostructure of the titanium dioxide layer is less. And, if the size of the silica particle exceeds 500 nm, there is a problem that the erosion of the silica layer is slowed down, and thus, the production efficiency of nanostructure of titanium dioxide layer is decreased. The size of the silica particle preferably used is 200˜300 nm. More preferable size of the silica particle is 210˜250 nm.

And, the silica particle preferably used in the present invention is a sphere.

The silica particle is synthesized by Stöber method, and the synthesized silica particle is washed and then is diluted with methanol without drying in order to avoid an aggregation of the silica particle to prepare a silica suspension for Langmuir-Blodgett process. The silica suspension is ultrasonic-stirred with sodium dodecyl sulfate under the heating to introduce a long hydrocarbon tail into the silica particle. The long hydrocarbon tails on the surface of this silica particle make the silica particles act as the amphipathic material. The ultrasonic treatment is conducted by adding chloroform to the silica particle suspension to which the hydrophilicity-hydrophobicity is imparted. The silica suspension thus treated drops into Langmuir-Blodgett trough that the substrate wherein titanium dioxide layer is formed is installed, and after the predetermined time, hexagonal close-packed (HCP) silica monolayer is transferred to titanium dioxide layer by raising the substrate to form the silica particle layer on the titanium dioxide layer.

In the Langmuir-Blodgett process, the surface pressure of the silica particle layer is preferably 10˜12 mN/m. This is to avoid the overlapping of the silica spheres. Preferred surface pressure is 11 mN/m.

Through the Langmuir-Blodgett process, the silica particle layer formed on the titanium dioxide layer is self-assembling monolayer.

FIG. 2( a) represents the π-A isotherm of SiO₂ bead monolayer prepared by using Langmuir-Blodgett trough. A phase transfer of the silica monolayer is achieved by the increase of the surface pressure, and this can be seen from the change of the isotherm slope. Also, the cross-sectional view of FIG. 2( a) is SEM cross-sectional image of HCP silica nanosphere monolayer obtained when the surface pressure is 11 mN/m. FIG. 2( b) represents the top view of SEM image of the silica nanosphere monolayer. As shown in FIG. 2( b), the monolayer of the silica particle is uniform and extended to a centimeter scale.

Then, the surfaces of the laminates of the substrate, titanium dioxide layer and silica particle layer prepared in the above step are etched.

In the method of tuning wettability of titanium dioxide layers against water, the etching is carried out by Inductively Coupled Plasma (ICP) etching method, plasma etching method or reaction-ion etching method.

ICP etching method is preferably used in the present invention, wherein the titanium dioxide layer on the substrate having the silica particle monolayer is etched as CF₄ (etchant), O₂ and Ar at the room temperature. In the present invention, the etching mode is an anisotropic mode, and the vertical etching ratio is much larger than the horizontal etching ratio. The etching procedure is carried out by a physical etching that kinetic Ar plasma ions collide with the sample ions. A chemical etching is primarily occurred by the reaction as follows:

CF₄=2F+CF₂  (1)

TiO₂+4F=TiF₄+2O  (2)

TiO₂+2CF₂=TiF₄+2CO  (3)

The predicted product of titanium dioxide layer is TiF₄ having higher boiling point of 284° C. at an atmospheric pressure.

Generally, upon referring to Wenzel state, the etched TiO₂ thin-film represents a smaller contact angle than that of the thin-films unetched. Wenzel equation is as follows:

cos θa=r cos θ  (4)

Wherein, θa is an apparent contact angle of water drop on the rough surface; θ represents an inherent contact angel on the surface of liquid droplet; r is a roughness factor which is defined as the ratio of the projected geometry to actual rough surface region. If θ is less than 90°, the surface will be more hydrophilic, and if θ is greater than 90°, the surface will be more hydrophobic.

The silica particle layer is etched during the etching procedure, wherein the titanium dioxide layer forms nanocones on the upper part based on the part that each of the silica particles is faced as the silica particle layer is etched. The cones formed from the uniform and ordered array.

The etching ratio of the silica is much faster than that of the titanium dioxide, and the etching ratio of SiO₂ particle is also considered in the formation of nanocones.

The preferable etching time in the present invention is in the range of from 10 to 40 sec. If the etching time is less than 10 sec, there is a problem that the etching efficiency of the silica particle is lowered and thus, there the nanocone formation is insufficient, and if the etching time exceeds 40 sec, there is a problem that the etchings of the titanium dioxide layer as well as the silica particle layer are severed and thus, the nanocones are disappeared. The preferable etching time is in the range of from 15 to 25 sec.

Then, the silica particle layer etched and remained in the stage is removed to form the nanostructured titanium dioxide layer.

In the present invention, the titanium dioxide layer constitutes the uniform and ordered nanocone array by removing silica remained on the titanium dioxide layer and thus, the nanostructuring thereof is completed.

In the present invention, the hydrophilicity of the titanium dioxide layer nanostructured as above is increased by modifying the surface. That is, the titanium dioxide layer nanostructured according to the method of the present invention has a smaller water contact angle compared to thin-film TiO₂ layer.

On the other hand, the hydrophobicity of titanium dioxide layer can be increased by forming the silane layer on the upper part of the titanium dioxide layer nanostructured in the step.

The silane layer in the present invention is preferably consisted of one or more of silane being selected from the group consisting of 1H,1H,2H,2H-perfluorooctylchlorosilane, octadecyltrichlorosilane, (tridecanfluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane, 3,3,3-trifluoropropyl-trichlorosilane, dodecyltrichlorosilane.

—CF₃ group has a low surface energy. If the surface energies of some functional groups are compared, they are as follows: —CH₂>—CH₃>—CF₂>—CF₂H>—CF₃.

Since the functional group having the low surface energy as above is formed on the titanium dioxide layer, the hydrophobicity of the titanium dioxide layer is increased.

In particular, the silane layer is formed on the upper part of the titanium dioxide layer nanostructured in the method of tuning wettability of titanium dioxide layer against water of the present invention, a sample in case of etching it for 15-25 sec represents hyper-hydrophobicity.

As such, in the present invention the titanium dioxide layer becomes hydrophilicity or hydrophobicity as the surface of titanium dioxide layer is modified, the wettability of the layer against water can be tuned.

If the method of tuning wettability of the titanium dioxide layer against water is used, the titanium dioxide structure having the titanium dioxide layer in a shape of nanocone array being uniformly arranged can be prepared.

The titanium dioxide structures of the present invention are characterized by comprising the titanium dioxide layer in the form of nanocone array uniformly arranged on the substrate. The titanium dioxide layer structures of the present invention increase the hydrophilicity thereof by nanostructuring of the titanium dioxide layer surface to the nanocone array shape.

Also, the silane layer can be coated on the upper part of the titanium dioxide layer. Wherein the silane layer is preferably consisted of one or more of silane selected from the group consisting of 1H,1H,2H,2H-perfluorooctylchlorosilane, octadecyltrichlorosilane, (tridecanfluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane, 3,3,3-trifluoropropyl-trichlorosilane, dodecyltrichlorosilane.

In the titanium dioxide structure of the present invention, if the silane layer is coated on the titanium dioxide layer, the hydrophobicity thereof is increased.

Hereinafter, the method of tunning wettability of titanium dioxide layers against water is reviewed in more detail through following examples and analyses.

Formation of TiO₂ Layer

1.1 Sol-Gel TiO₂ Thin-Film

TiO₂ thin-film was synthesized by using ethanol (Merck, 99.8%), Titanium Tetraisopropoxide (TTIP) (Daejung Chemical, 98% or more) and nitric acid (Daejung Chemical, 60˜62%) in the sol-gel method: 0.5 ml TTIP was mixed with 4.5 ml ethanol, and then 10 μl nitric acid was added to the mixture to prepare TiO₂ sol while inhibiting the precipitation of TiO₂ powder. The TiO₂ sol was applied to ultrasonic treatment for 30 min, and then white TiO₂ powder precipitate was filtered. Sol-gel TiO₂ layer having 85 nm thickness was formed over the silicon substrate by a spin coating (3000 rpm, 18 s) by using the filtrate solution filtered as above.

1.2 Sputtering TiO₂ Thin-Film

TiO₂ thin-film was prepared by vapor-depositing TiO₂ on the silicon substrate by using a multi-target co-sputtering technique: The sputtering ratio of TiO₂ and Ti is maintained as 3:1. Then, the thin-film was heated in a 400° C. furnace which is at an ambient air for 4 hrs. The thickness of the sputtered thin-film was 250 nm.

1.3 Thermooxidation TiO₂ Thin-Film

250 nm Ti layer was formed on the silicon wafer by using E-beam evaporator, and then heated in a 500° C. furnace which is at an ambient air for 8 hours to oxidate the Ti layer to form a thermooxidation TiO₂ thin-film.

The following procedures were progressed by using three types of TiO₂ thin-film prepared in Example 1.

Synthesis of SiO₂ Particle

Silica particles were synthesized by the Stöber method by using 20 ml ethanol, 3.29 ml distilled water, 0.55 ml nitric acid (Daejung Chemicals, 25˜28%) and 2.3 ml tetraethylorthosilicate (TEOS) (Aldrich). The synthesized silica particles (spheres) are mono-dispersed, and the mean size thereof was 225 nm.

Formation of Hexagonal Close Packed SiO₂ Monolayer and the Coupling with the Titanium Dioxide Layer

The silica particles prepared from Example 2 were washed four times with ethanol, and then diluted with 20 ml methanol (Daejung Chemicals, 94%) without drying to prepare the silica suspension to avoid the aggregation of the silica particles and to use Langmuir-Blodgett technique. 2 ml of silica suspension was used. To the silica suspension 2.5 mg of sodium dodecyl sulfate (Aldrich) was added, and they are ultrasonic dispersed while heating for 60 min to impart the hydrophilic-hydrophobic property to the silica spheres. Then, to the silica suspension 3 ml of chloroform (Junsei, 99%) was added and the ultrasonic treatment was applied to that for 60 min again. In Langmuir-Blodgett trough having an initial water area of 300 cm², the substrate that TiO₂ thin-film prepared from Example 1 was formed was installed. To the Langmuir-Blodgett trough 250 μl of the silica suspension was dropped. Before the isotherm process, a waiting was made for 20 min until the initial surface pressure was stabilized so that all the solvents are vaporized. Thereafter, the self-assembling silica monolayer was pressured with the barrier speed of 20 cm²/min until the target surface pressure was 11 mN/m. The hexagonal close packed SiO₂ monolayer was formed on the upper part of titanium dioxide thin-film by elevating the vertical substrate that the TiO₂ thin-film was formed with the speed of 1 mm/min and thus, transferring the hexagonal close packed SiO₂ monolayer to the upper part of the titanium dioxide thin-film. The transferring ratio was about 1.

Etching

The laminate that the hexagonal close packed SiO₂ monomolecular layer was formed on the titanium dioxide thin-film prepared from Example 3 was etched by using the mixed gas of CF₄, O₂ and Ar by ICP dry etching method at the room temperature. After the etching procedure, the samples were washed by stirring them in the distilled water with the sonicator during 5 min, and then washed them in pure ethanol by the same method to remove any residual silica on the surfaces of the samples.

Silanization of Surface of TiO₂ Layer

Unetching TiO₂ thin-film sample and TiO₂ thin-film samples which completed the procedures until Example 4 were dipped into n-hexane comprising 0.5% 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFTS) (Aldrich) for 10 min to coat the surfaces of the thin-film samples with PFTS. Then, they are carefully washed with hexane to remove any residual PFTS. After drying the thin-film samples at an ambient air, they are treated for 1 hour in 110° C. vacuum oven.

Analysis

The shapes and thicknesses of the samples as prepared from the Examples were measured by SEM (FE Type) XL30SFEG (PHILIPS) and UHR-SEM Magellan 400(FEI). X-ray photoelectron spectroscopy (XPS) spectrum was taken on Sigma Probe system (Thermo VG Scientific) equipped with Al—Kα X-ray source (1486.3 eV) and 0.47 eV FWHM of energy resolution under UHV condition OF 10⁻¹⁰ Torr. The XPS spectrum was fitted by using CASAXPS software. Water contact angle was measured on the thin-film samples by using a goniometer (Phoenix 300 plux, SEC)). In order to achieve the immobilization of water drops, the water contact angle was measured at 1 min after making water drops contact with the surfaces of the thin-film samples.

(1) Comparison of Shape of Sputtering TiO₂ Thin-Film Over the Etching Time

FIG. 3 shows the top and cross-sectional views of Scanning Electron Microscopy (SEM) image of sputtering TiO₂ thin-film sample. FIG. 3( a) shows the images at (a) unetching, (b) 10 sec etching, (c) 15 sec etching, (d) 20 sec etching, (e) 25 sec etching and (f) 30 sec etching. As the etching time increases, the silica nanospheres were increasingly etched to disappear, in particular it can be ascertained that 225 nm of SiO₂ is mostly etched after 30 sec etching as shown in FIG. 3( f).

(2) Comparison of the Height of TiO₂ Nanocone According to Etching Time Change of Sol-Gel/Sputtering/Thermooxidation TiO₂ Thin-Films

Table 1 suggests the height change of TiO₂ nanocone according to the etching time change of sol-gel/sputtering/thermooxidation TiO₂ thin-film.

After 20 sec etching, the height of sol-gel TiO₂ nanocone was ˜104.8 nm, which was almost 2 times over that of sputtering TiO₂ nanocone being ˜59.7 nm and that of thermooxidation TiO₂ nanocone being ˜54.6 nm. Thus, it could be seen that sol-gel TiO₂ thin-film sample has more effect on etching than other TiO₂ thin-film samples.

Also, upon 30 sec etching, the height of sol-gel TiO₂ nanocones was greatly decreased to ˜47.2 nm, whereas the heights of sputtering and thermooxidation TiO₂ nanocones were 89.8 nm and 72.3 nm, respectively. That is, if SiO₂ sphere mask is disappeared, since the upper part of titanium dioxide nancone is not shielded, it could be seen that the surface part having a more etching ratio is faster.

In case of sol-gel TiO₂ thin-film, upon etching for 30 sec or more, most of the titanium dioxides are etched to expose the silicon substrate beneath the titanium dioxide layer. Eventually, upon etching for 40 sec, all of TiO₂ layers were removed.

Further, if the sputtering and thermooxidation TiO₂ thin-film were etched for 40 sec, it could be ascertained that there was a decrease in the height of titanium dioxide nanocone than that of 30 sec etching. Also, it could be ascertained that the thermooxidation TiO₂ thin-film endures the etching better than the sputtering TiO₂ thin-film.

TABLE 1 Height of titanium dioxide nanocone (nm) Etching time Thermooxidation (s) So-gel TiO₂ Sputtering TiO₂ TiO₂ 10 54.6 33.2 30.3 20 104.8 59.7 54.6 30 47.2 89.8 72.3 40 — 76.8 70.4 (3) Relation Between the Contact Angle and the Etching Time Before and after the Silanation of the Surface of TiO₂ Layer

FIG. 4 depicts a graph representing the relation between the etching time and the water contact angle of TiO₂ layer before (a) and after (b) the silanation by using 1H,1H,2H,2H-perfluorooctyltrichlorosilane.

FIG. 4( a) shows the relation between the etching time and the water contact angle of TiO₂ layer not silanated on the surface, and represented the features that the contact angle is decreased as the etching time is increased overall of sol-gel, sputtering and thermooxidation TiO₂ thin-film samples. That is, as the etching is progressed, it could be ascertained that the hydrophilicity of titanium dioxide layer was increased by the nanostructuring of titanium dioxide layer. In case of sputtering TiO₂ thin-film sample, the value was represented that the water contact angle was slightly increased upon 30 sec etching, but this may be caused from a contamination of sample surface.

FIG. 4( b) represents the relation between the etching time and the contact angle after coating the silane layer on TiO₂ layer surface, and wherein the unetching, sol-gel, sputtering and thermooxidation TiO₂ thin-film samples represented the water contact angle of nearly 100°. In case of the samples etched during 10 sec and 20 sec, it represented that the water contact angle was rapidly increased in sol-gel TiO₂ thin-film sample, and the angle was moderately increased in sputtering and thermooxidation TiO₂ sample, and these coincide with the height growth of titanium dioxide of Table 1.

(4) Comparison of Shape and Contact after the Silanation of 20 Sec Etching Sol-Gel/Sputtering/Thermooxidation TiO₂ Thin-Films

FIG. 5 is the cross-sectional and top views of SEM image and photoimage of water contact angle after the silanation of 20 sec etching TiO₂ thin film, wherein (a) is a sol-gel method, (b) is a sputtering method, (c) is a thermooxidation method TiO₂ layer. Sol-gel TiO₂ thin-film had the contact angel of 155° upon 20 sec etching and represented the super-hydrophobicity, and the contact angles of sputtering TiO₂ thin-film and thermo-oxidation TiO₂ thin-film were 130° and 138°, respectively.

(5) Relation Between the Water Repellency and the Geometrical Structure of Titanium Dioxide Nancone

The lower surface energy of —CF₃ group together with air trapped beneath the water drops due to the geometrical structure of titanium dioxide nanocone increases the water repellency. This result coincides with Cassie and Baxter equation, and the wetting of heterogeneous surface is represented by the following equation:

cos θa=Σfi cos θi  (5)

Wherein θa is an apparent contact angle of water drop, fi is an area part of the constitutional component i, and the contact angle of i on the plane is θi. Wherein the sample to be used can be considered as 2-component system consisted of titanium dioxide and air, having the water contact angle of 180° in air (non-wetting). Since Σfi=1, the equation (5) is as follows:

cos θa=f1(cos θ1+1)−1  (6)

Equation (6) proved that when f1 is sufficiently low, since the apparent contact angle (θa) reaches to nearly 180° and thus cos θa is close to −1, if a larger number of air is present below the water drop, it becomes more hydrophobic surface. Sol-gel TiO₂ thin-film samples have the contact angle of 155° and become super-hydrophobic whereas the sputtering and thermooxidation TiO₂ thin-film samples represent considerably strengthen water contact angle compared to the plane sample and 10 sec etched sample.

In case of the sample etched longer than 20 sec, although the sputtering method and thermooxidation method TiO₂ nanocones reached to the highest height and have higher air area higher below the water drop, the decrease in the water contact angle of three types of titanium dioxides due to the bigger space between the nanocones was found. In order to explain such certain circumstance, the geometric surface structure should be considered.

Sputtering and thermooxidation titanium dioxide nanocone array formed after 20 sec etching represents the structure of inclined side wall. As described above, the solid phase, liquid phase and gas phase complex interface as the air pocket trapped below the water drop further represents the super-hydrophobicity. Consequently, meniscus along the inclined side wall is allowed to form and maintain the complex interface, based on Laplace pressure. Younghao Xiu et al. suggested the role and mechanism of Laplace pressure. The relation between the Laplace pressure and the inclined angle is given by the following equation:

${\Delta \; p} = {{p - p_{0}} = \frac{\gamma \; {\cos \left( {\theta - \alpha} \right)}}{R_{0} + {h\; \tan \; \alpha}}}$

Wherein Δp, p, and p0 represent the Laplace pressure, the pressure and atmospheric pressure of the liquid side of the meniscus, respectively; γ represents the surface tension of water; θ represents Young's water contact angle on the surface; α represents the inclined angle; R₀ represents a half of the width between bottom sides of two adjacent side walls; h represents the distance between the meniscus and the floor. The equation represents the dependency of the inclined angle to the Laplace pressure. If the inclined angle is small, Laplace pressure is high, a larger amount of air is trapped beneath the water drop.

Upon etching for more than 30 sec, SiO₂ nanosphere monolayer on the titanium dioxide layer surface is completely etched to disappear. If all the silica particles are removed, the upper part of titanium dioxide is not protected any more, and the etching is occurred more rapidly at a higher part of the surface. Thus, after 30 sec etching, nanocone sides are inclined more than after 20 sec, and the water contact angle of 30 sec etched TiO₂ thin-film samples is smaller. Also, due to the inherent roughness of sputtering method and thermooxidation method TiO₂, the pyramid-shape structure of the upper part of the nanocone may be the factor increasing the water contact angle of the samples. On the other hand, the pyramid-shape structure can be replaced with a stepped structure on the surface of 30 sec etching nanocone. The explanation on this is shown in FIG. 6, and can be used in explaining that the water contact angle of the sputtering method and thermooxidation TiO₂ etched for 40 sec is more lowered. Due to insufficient thickness of sol-gel TiO₂ thin-films, the water contact angle on the sol-gel TiO₂ film etched for 30 sec with the free-titanium dioxide on each of silicon cones is considerably reduced, and this is also explained similarly to the sputtering and thermooxidation TiO₂ films. The high inclined angle of the silicon cones also explains the reduced water contact angle of 30 sec etched sol-gel TiO₂ thin-film sample.

(6) Confirmation of Etching Product Upon Etching of the Titanium Dioxide Layer

XPS spectrum of etched TiO₂ layer is decipted in FIG. 7. (a) and (b) are Ti component and F component of 20 sec etching sol-gel TiO₂ layer, respectively, and (c) and (d) are Ti component and F component of 40 sec etching thermo-oxidation TiO₂ layer, respectively.

According to this, TiF₄ might be not formed practically in the etching process; fluorine atoms can physically adsorb on surface, or occupy and substitute the positions of oxygen atoms, behave as dopants in TiO₂ structures. The fluorine dopants are able to improve photocatalytic activity of titanium dioxide.

Although the concrete Examples of the present invention are given as above, the present invention is not limited to the above and can be variously modified within the technical scope of the present invention and this modification belongs to the claims of the present invention, as indicated below. 

What is claimed is:
 1. A method for forming a nanostructured titanium dioxide layer, the method comprising: (a) step of forming a titanium dioxide layer on a substrate; (b) step of forming a number of particle layers on the upper part of the titanium dioxide layer as an etching mask; (c) step of etching a surface of the titanium dioxide layer as an etching mask to form a nanostructure on the surface of the titanium dioxide layer by the etching; and (d) step of removing the residual particle layer.
 2. The method for forming a nanostructured titanium dioxide layer according to claim 1, wherein the particle layer is a silica particle.
 3. The method for forming a nanostructured titanium dioxide layer according to claim 2, wherein the titanium dioxide layer in the step (a) is formed by a sol-gel method, sputtering method or thermooxidation method.
 4. The method for forming a nanostructured titanium dioxide layer according to claim 2, wherein the silica particle layer in the step (b) is formed by Langmuir-Blodgett process.
 5. The method for forming a nanostructured titanium dioxide layer according to claim 4, wherein the surface pressure of the silica particle layer in the Langmuir-Blodgett process is 10˜12 mN/m.
 6. The method for forming a nanostructured titanium dioxide layer according to claim 4, wherein the silica particle layer is a self-assembling monolayer.
 7. The method for forming a nanostructured titanium dioxide layer according to claim 2, wherein the etching in the step (c) is conducted by ICP etching method or reaction-ion etching method.
 8. The method for forming a nanostructured titanium dioxide layer according to claim 7, wherein the etching is conducted for 10-40 sec.
 9. The method for forming a nanostructured titanium dioxide layer according to claim 2, wherein after conducting the step (d) a silane layer is formed on the surface of the nanostructured titanium dioxide layer to increase the hydrophobicity.
 10. The method for forming a nanostructured titanium dioxide layer according to claim 9, wherein the silane layer is consisted of one or more of silane being selected from the group consisting of 1H,1H,2H,2H-perfluorooctylchlorosilane, octadecyltrichlorosilane, (tridecanfluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane, 3,3,3-trifluoropropyl-trichlorosilane, and dodecyltrichlorosilane.
 11. In a structure that a control of wettability against water is possible, the structure comprising: a substrate; and a titanium dioxide layer formed on the substrate, wherein the titanium dioxide layer has a surface on which a number of nanostructures in a cone shape is formed.
 12. The structure according to claim 11, further comprising a silane layer that is coated on the upper part of the titanium dioxide layer. 